The exchange part of a mass transfer column in which, in particular, an upwards flowing gas and a counter-flowing liquid are brought into contact, consists of a packing which fills the cross-section of the column out to an annular gap at the column wall. The crown elements which encircle the packing serve for the deflection of these flows of matter. The gas flow is forced into the packing by the crown elements; the gas is thereby impeded from partially flowing through along the region at the edge of the column without coming into contact with the liquid. The packing is in particular an ordered packing which brings about a distribution of the two flows of matter across the cross-section of the column. Such a crown element is for instance disclosed in U.S. Pat. No. 5,456,865 or JP08-000983 A.
The packing is composed, for example, of layers of undulating corrugations or laminations folded in a zigzag. The corrugations consist of a material like foil; the edges at the folds or ridges extend in a direction inclined with respect to the axis of the column, for example, at an angle to the axis amounting of 45 DEG or 30 DEG. The layers formed by the corrugations are aligned in parallel with the axis; with respect to the slope of the edges at the folds they are arranged alternately so that open channels result which run obliquely and cross. The packing is organized in sections which differ in the orientation of the layering. Between adjacent sections the orientation changes each time through approximately 90 DEG.
Through the oblique channels in the packing, part of the liquid flows to the region at the edge. To prevent this liquid from contacting the column wall an edge gap is provided, that is, the above-mentioned annular gap. Without the edge gap a considerable part of the liquid would flow down along the column wall.
By means of the edge gap, it becomes possible for the liquid arriving at the edge of the packing to be returned into the interior of the packing again through other channels. However, it cannot be avoided in practice that a certain proportion of the liquid does spill over against the column wall and thereby impair the mass transfer process. The crown elements have the additional function of conducting the liquid back from the column wall to the packing where the liquid is subsequently led through some of the channels back into the interior of the packing.
Further examples of ordered packings, which bring about a distribution of the two flows of matter across the cross-section of column, are described in the European Patents EP 0 070 917 B1 and EP 0 069 241 B1.
A packing of a mass transfer column comprises at least one packing element. A plurality of packing elements can be stacked upon each other to form a stack. A packing element is defined as an assembly of packing layers forming a substantially cylindrically shaped structure or any other structure fitting into the interior of the mass transfer column. Such a packing element can be subdivided into a plurality of packing segments. The packing segments can thus be considered as the slices forming a packing element.
For simple mounting of the column, it is necessary for the crown elements (i.e. collars) to be fastened to the built-in elements already before installing the packing. Since the packing consists of thin-walled material, the fastening of the collars, which may be done by welding, screwing or riveting is an expensive manufacturing step.
When using packing elements without fixed collars attached to each boundary element of the packing a sealing has to be guaranteed by a loose collar. When about to be assembled, the loose collars are first placed horizontally along the column wall, then the packing elements are mounted. Finally the packing elements are pushed from a central position against the wall. Thereby the collar is stretched towards the wall and therefore put under tension by the compression force resulting from the packing. The loose collars according to the prior art are not flexible and consequently do not provide an adequate sealing. This has the consequence that in operation a liquid by-pass occurs next to the column wall, which results in a decrease of the column performance. Due to their geometrical arrangement, the loose collars may be flattened and destroyed in particular, when the packing has to be assembled into a column wall, which has a horizontal longitudinal axis.